Iranian authorities have arrested a U.S. citizen whose son has been detained in Iran for the last four months, friends and family said Wednesday, describing what would be the first such action against an American national in Iran since last month’s prisoner swap between the two countries.

Baquer Namazi, whose son Siamak has been held since October, was arrested on Monday, his wife wrote in a Facebook post. She said she was told he is now in Tehran’s Evin prison.